1. Field of the Invention
This invention pertains to an improvement in the efficiency with which oil and gas are recovered from an oil and gas reservoir by introducing a silicone-containing composition comprised of silicone and an organic solvent into the oil and gas reservoir.
2. Description of the Prior Art
Oil, gas and water exist in varying degrees in oil and gas reservoirs where retentive forces derived from interfacial tensions between these different reservoir fluids and the reservoir rock preclude the recovery of all oil and gas present. These hydrocarbons and water are contained under pressure within the reservoir. Recovery of these reservoir fluids is accomplished by completing a well in the reservoir and using existing or artificially created pressure differentials to drive the reservoir fluids toward the low pressure area caused by the well.
Pressure upon the reservoir by overlying earth strata, hydrodynamic or hydrostatic forces of encroaching edge water, gravity, the resilient expansion of the compressed strata, and the forces of interfacial phenomena cause the compaction of the natural gas and oils. Natural gas is dissolved in the oil, and to a much lesser extent may be dissolved in the water or adsorbed on the mineral surfaces of the reservoir rock, thereby reducing the area occupied by the reservoir fluids.
Oil is capable of holding large quantities of natural gas in solution, where saturation is determined by temperature and pressure subjected on the reservoir in addition to the molecular composition of the hydrocarbons.
The "free gas", that which is not dissolved in oil or water, generally accumulates in one or more gas caps located above the oil-gas interface in the reservoir rock. The heavier oil settles to the lower areas of the reservoir. Water is invariably found in the oil and gas reservoir. This water is either connate water retained by capillarity within the smaller interstices of the rock during the formation of the reservoir, or water that has accumulated where the gravitational segregation of the reservoir fluids is incomplete.
The completion of a well in the reservoir creates a low pressure area. The reservoir fluids expand to fill this low pressure area until an equalibrium is reached. There are several "drive" forces causing this migration as well as other retentive forces that must be overcome before movement will take place. The major drive forces are gravity; "depletion drive", caused by the expansion of the free gas trapped at high points or caps; "solution drive", caused by release and expansion of natural gas from the oil and water; and "water drive", produced by the hydrodynamic or hydrostatic force of encroaching edge water. Interface phenomena, such as surface free energy, surface tension, interfacial tension, adsorption, adhesive forces, pore friction during fluid movement, and capillarity play important roles in the retention and movement of reservoir fluids.
Maximum efficiency of oil recovery depends on numerous factors including oil viscosity, rate of recovery, retentive forces acting upon the reservoir fluids, the composition of the reservoir rock, and especially maintenance of both a pressure gradient and sufficient oil saturation at the well.
The efficiency rate for oil recovery by solution drive when used alone is quite low, estimated at being between 10 and 30 percent. This low recovery rate is attributed to the limited amount of gas available and the high gas-oil recovery ratio commonly present. This high gas-oil recovery ratio has three bases. First, the natural gas with greater mobility than the oil due to its lower viscosity, forms channels through the oil directly to the well thereby carrying less oil during expulsion from the reservoir. Secondly, the most efficient production rate is often too slow to profitably produce the oil, therefore production is increased by allowing the pressure gradient within the well to drop rapidly. Third, as the gas is released from the oil in the form of tiny bubbles the oil viscosity is temporarily lowered, but as the gas bubbles consolidate the phase continuity of the remaining oil increases as does the oil viscosity making the oil more resistant to movement.
The depletion drive effect of trapped free gas extends capillary pressure that drives the reservoir fluids from pore to pore toward the low pressure area around the well. As the reservoir fluids are extracted from the well, a void is created that is filled by the reservoir fluids driven by the expanding natural gas. Professor Lester Charles Uren in his book entitled Petroleum Production Engineering (1953) pictures this void as being filled in the early stages of production by a gas-oil froth in which the gas is imprisoned within the oil films in the form of bubbles. This froth is unstable and eventually gravitational settling of the oil occurs with the heavier more viscous oil tending to occupy the lower portions of the strata to which it has access.
As the pressure in the well, and thereby the reservoir, drops, somewhere along the pressure gradient the bubble point is reached and tiny bubbles of natural gas are released from the oil and begin to expand. These bubbles are pictured by Uren as linking together to form multitudes of tiny channels capable of quick dissemination of natural gas to tangential low pressure areas. These bubbles, enclosed by an oil film, sometimes break permitting two or more to join forming a larger bubble; or a larger bubble may be broken into a multiplicity of smaller bubbles when necessary to pass through smaller pore spaces. These gas bubbles, however, continue to carry some oil with them, and are delivered into the well largely in the form of films on gas bubbles: a froth rather than a liquid. If the reservoir pressure drops considerably below the bubble point, released gas with its lower viscosity expands to occupy more and more area proximate to the well. The amount of oil that clings to the natural gas will decrease as the volume of gas near the well increases. Further, the formation of channels of natural gas tends to deliver the natural gas to the well without displacing or carrying much oil. This precipitates a high gas-oil ratio and low recovery efficiency.
In replacing oil and gas in the reservoir, encroaching edge water acts to maintain the pressure gradient within the reservoir while flushing the reservoir fluids out of the rock pores and driving these fluids toward the well. Where an active water drive is available, there is usually an abundance of energy present, such that if gas and oil are produced too fast the well can be shut down for a period of time and the pressure will be replenished. Water drive, therefore, has a higher efficiency rate than depletion or solution drive forces when used alone, but even under the best circumstances an estimated 20 percent of the recoverable oil remains within the pool with most pools leaving over 40 percent behind.
These drive forces may act in concert, but usually one drive force will dominate at any given time. The recovery efficiency may be improved through the use of applied artificial force, such as by injection of gas or water flooding. This form of pressure maintenance or repressurization is often accomplished by conventionally pumping the desired gas or water into the reservoir through neighboring wells.
Other methods of secondary recovery include in situ combustion, heat injection, or the use of a foam to drive the reservoir fluids to the well. However, all of these applied artificial forces, with the exception of in situ combustion, which is very inefficient anyway, do not take substantial advantage of the existing energy available through release of dissolved or entrained natural gas present in the oil.
It has long been recognized that liquid organo-silicon and organo-silicate condensation products or polymers may be employed an anti-foaming agents in hydrocarbon oils which tend to foam or froth when agitated and/or are exposed to various gases. Such materials are preferred an anti-foaming agents since they have little effect on the generally desired properties of oil. However, in spite of such long available knowledge, it has not previously been recognized that such silicone-containing anti-foaming agents were useful in pressure maintenance in secondary recovery action to obtain oil from oil and gas reservoirs.
Therefore, it is a feature of the present invention to provide an improved process of providing pressure maintenance in secondary recovery action to obtain oil from oil and gas reservoirs.